1. Technical Field
The present invention relates to organic electroluminescent devices, methods for producing the devices, and electronic apparatuses.
2. Related Art
The need for lightweight, low-power-consumption flat displays is growing with the increasing variety of information devices. One such known flat display is an organic electroluminescent (EL) device, which includes an organic light-emitting layer.
An organic EL device includes light-emitting elements formed using organic materials. The basic structure of the light-emitting elements includes an organic light-emitting layer (light-emitting layer) held between an anode and a cathode. In addition to this basic structure, the light-emitting elements often include functional layers with various functions, including a hole-injecting layer disposed between the anode and the light-emitting layer to facilitate injection of holes from the anode and an electron-injecting layer disposed between the cathode and the light-emitting layer to facilitate injection of electrons from the cathode. These functional layers contribute to high luminance and high light-emission efficiency.
Most materials used for the layers such as the light-emitting layer, the hole-injecting layer, and the electron-injecting layer tend to be degraded because they react easily with moisture and oxygen in air. If the layers are degraded, non-light-emitting regions called “dark spots” are formed in the organic EL device, thus lowering the performance of the light-emitting elements. Accordingly, some structures for sealing the light-emitting elements with a gas barrier layer such as a moisture-resistant thin film or seal have been proposed to prevent intrusion of moisture and oxygen into the organic EL device (for example, see JP-A-2001-284041, JP-A-2005-293946, JP-A-2006-150592, and JP-A-2007-35549).
A small or medium-sized display for use as a display unit of a cellular phone has a non-display region around a display region, called a frame region, which is typically narrowed to a width of not more than 2 mm to leave a larger effective display region for functionality and design purposes. Accordingly, an organic EL device for use in a cellular phone requires a narrow-frame structure with a narrow frame region. To prevent intrusion of moisture into the device, on the other hand, it is effective to form a gas barrier layer (seal layer) using a material with low moisture permeability, as described above, and the periphery of the frame region is a suitable site where the seal layer is formed. There is difficulty, however, in allocating the entire frame region to the seal layer in terms of panel design; other components with various functions, including a cathode line, are stacked on top of each other in the region where the seal layer is disposed.
The cathode line is connected to an electrode (cathode) of the light-emitting elements constituting the display region to provide an electrical connection to a connection terminal. It is therefore desirable to form the cathode line using a material with a lower electrical resistance than the cathode of the light-emitting elements to facilitate conduction. In addition, the cathode line is generally widened so that it has a smaller sectional area and therefore a lower electrical resistance. To electrically connect the cathode line to the cathode of the light-emitting elements from near the light-emitting elements, additionally, the cathode line is often formed in the frame region so as to surround the display region where the light-emitting elements are disposed.
The cathode line, usually formed of a metal with high electrical conductivity, such as aluminum, titanium, molybdenum, tantalum, silver, copper, or an alloy thereof, is typically covered with an indium tin oxide (ITO) conductive film used for the pixel anodes. The cathode line has a problem in that it may be corroded by an etchant (a strongly acidic solution such as iron chloride/hydrochloric acid or nitric acid/sulfuric acid) used in a subsequent patterning step for forming the pixel electrodes (ITO); patterning by etching in a photolithography step is essential in forming wiring or pixel portions in a production process. This problem is solved by covering the entire surface of the cathode line with the ITO layer and a photoresist layer for protecting the ITO. As a result, the ITO used to form the pixel electrodes is also deposited on the top surface of the cathode line.
A detailed study of the structure and moisture resistance of the frame region by the inventor, however, has found that ITO can be permeable to moisture, depending on the deposition method used, and moisture may intrude into the device through the ITO over the cathode line. For known device structures, the cathode line is often exposed in the frame region because it has not been believed that wiring can form a path through which moisture intrudes. In addition, the cathode line is typically wider than other wiring and is formed near the light-emitting elements for the reason described above. Once moisture intrudes through the cathode line, therefore, it may easily reach and degrade the light-emitting elements. The possibility of moisture intrusion through the wiring is not described in any of the above patent documents, in which no measure is suggested accordingly.